We shrink at that question. If forced to answer, some reply: "All human life is of infinite value." Others cite the Jewish teaching that if you put one human life on one side of a scale, and the rest of the world on the other side, the scale is balanced equally. Most just say that an Afghan life is worth the same as a British life, because all human lives are of equal value. Isn't that what we all believe?

The Ministry of Defence has been paying compensation to Afghans for accidentally killing their children, their brothers and sisters, or their parents, during the fighting in Afghanistan. Thanks to a freedom of information request from the Guardian, we know how much the MoD has paid families when a member has been killed. Here are some examples: daughter hit by shrapnel from air-strike and later died of injuries, $1,000; mother killed during bombing, $5,000; two brothers and two sons killed by hellfire missile strike, $32,000. The variation in the figures is not explained, but in no case was more than $8,000 (about £5,000), paid for the loss of a single life.

Now let's take a look at the value of a British life. The National Institute for Health and Clinical Excellence (Nice) assesses drugs and other medical treatments for cost-effectiveness and recommends whether they should be supplied on the National Health Service. Nice is commendably transparent. No freedom of information requests are needed. Just visit its website for a description of how it decides if a treatment is worth paying for. The key factual question is how much the treatment costs for each quality-adjusted life year, or QALY, gained. A QALY is one year of life of good quality, or its equivalent, which might be a longer period of life of lower quality. The website then tells you that while decisions are made on a case by case basis, "generally … if a treatment costs more than £20,000-30,000 per QALY, then it would not be considered cost effective".

Remember, that sum is per QALY, not per life saved. So if we take the bottom end of this range, Nice recommends that the NHS pay up to four times as much to extend the life of a British citizen by just one year, as the MoD is prepared to pay in compensation for killing a child or young person. That young person could – even allowing for Afghanistan's dismal life expectancy – expect to live another 40 reasonably good-quality years. That suggests an answer to the question with which I started: it takes about 4 x 40, or 160 Afghan lives, to be worth the same as one British life.

But that would not be the right answer, because £5,000 will buy much more in Afghanistan than it would buy in Britain – according to international price comparisons, perhaps four or five times as much. Let's say five times. Even with that adjustment, it is going to take 32 Afghan lives to be worth the same as one British life.

There is nothing unique about Britain in this respect. The Guardian has reported that the US generally pays no more than $2,500 in compensation for the loss of an Afghan life. In contrast, after the terrorist attacks of 11 September 2001, the US government set up a Victim Compensation Fund. The average payment it made to families of victims was $1.8m. Adjusting for purchasing power at a 5:1 ratio suggest that the US regards the life of an American as equivalent to the lives of 144 Afghans.

What would happen if the Nato forces really took seriously the idea of the equal value of all human life? They would then have to compensate Afghans for the civilian deaths and injuries they are causing at the same level as they would compensate their own citizens if, for example, a military exercise went wrong and killed people at home. That would serve three important purposes. First, it would demonstrate to the Afghans that the Nato forces truly respect them as equals. Second, the troops themselves might start to see Afghans as more like them, and have a new respect for the people they are trying to aid. Third, a dramatic increase in the costs of endangering the lives and limbs of civilians might foster a new restraint, because no military force wants to drain its own resources. The result would then be that fewer civilians would be killed – surely a very good thing, both for the civilians themselves, and for winning over the support of Afghans.